1. Field of the Invention
The present invention relates to a process for preparing a laminate of a metal and a polyolefin resin. More specifically, the invention relates to a process for preparing a laminate which excellently withstands against the deterioration of adhesion caused by aging.
2. Description of the Related Art
The surfaces of metals have heretofore been coated with a resin to protect them from corrosion, fouling and damage. For instance, a transition joint that is used for the insert molding is subjected to severe environment in which it is brought into contact with molten resins. Therefore, its metallic threaded portions must be coated with a resin. A variety of resins for coating use are suitably selected depending upon the applications and the cost required.
The coating with resin is not limited to the above applications only but is widely employed in such fields as various parts and structural members made of metals, such as commodities, packaging materials, various panels, interior finish members, various casings, etc.
The polyolefin resin can be used for general purposes exhibiting such advantages as easy melt-adhesion upon heating, resistance against water, resistance against humidity, and excellent sanitary properties, and is further, available relatively cheaply. Because of its poor polarity, however, the polyolefin resin is adhered to metals only poorly and peels off easily from the metal after it is once adhered.
In order to improve this defect, an anchor coating agent of organotitanium or the like is applied in advance to the surface of the metal which is to be coated with the polyolefin resin, and then the polyolefin type resin is heat-adhered via the anchor coating agent, as is widely known.
Moreover, it has heretofore been known to improve the adhesiveness by introducing a polar group into the polyolefin resin, and to graft-modify the polyolefin resin with an unsaturated carboxylic acid or an anhydride thereof such as a maleic anhydride in order to apply and heat-adhere it onto the surface of the metal substrate. For instance, according to Japanese Patent Publication No. 10184/1981, a polyolefin grafted with an unsaturated carboxylic acid or an anhydride thereof is heat-adhered to a metal fitting via an organosilicon compound that has a hydroxyl group or a hydrolyzable group and an acid-reactive group and that is disposed on an interface between the two. Moreover, Japanese Patent Publication No. 12299/1983 discloses a composition consisting of 70 to 95 parts by weight of an acid-modified polyethylene and 5 to 30 parts by weight of a polypropylene, which is laminated on a metal, and Japanese Patent Publication No. 40491/1983 discloses a composition consisting of 99 to 70 parts by weight of an acid-modified polyolefin and 1 to 30 parts by weight of a hydrocarbon-type elastomer, that is used as an adhesive agent for laminating a polyolefin on a metal foil.
In fact, however, though the laminate of a polyolefin type resin and a metal substrate exhibits a high adhesion strength during the initial period, the adhesion strength decreases with the lapse of time almost without exception. This tendency appears conspicuously in an environment where water is present. For instance, the aforementioned acid-modified polyolefin resin exhibits a considerably great initial adhesion strength for metals but loses adhesive force drastically with the lapse of time in an environment where water exists. The tendency of adhesion deterioration is also recognized even when the surface of the metal is treated with an anchor coating agent such as organotitanate prior to effecting the adhesion.
In preparing a laminate by coating a metal substrate with a polyolefin resin and heat-adhering it thereon, therefore, it is an object of the present invention to provide a treatment method which prevents the adhesion strength between the metal substrate and the polyolefin resin from decreasing with the lapse of time and, particularly, which prevents the adhesion strength from decreasing in an environment where water exists.
Another object of the present invention is to provide a method of effecting the treatment for preventing the adhesion strength from decreasing with the lapse of time in a simple and easy manner at a relatively low cost.